1. Field of the Invention
The present invention relates in general to plasma reactors having an RF applicator and more particularly to a method and an apparatus for actively controlling the density of the species generated within such a plasma reactor using time-modulation.
2. Background Art
Plasma (or xe2x80x9cdryxe2x80x9d) etch processing is vitally important to several of the largest manufacturing industries in the world. In particular, plasma etch processing is indispensable in the manufacture of semiconductors, integrated circuits and microelectronic devices. These products are essential to numerous industries including the computer, electronics, biomedical and aerospace industries.
Plasma etch processing is used to modify the surface properties of a workpiece. For example, in semiconductor fabrication the workpiece is a semiconductor wafer and the plasma etch process removes patterned material from the surface of the wafer. Etch requirements such as etch depth may vary between wafers depending on, for example, the hole depth. Thus, a wafer having one pattern usually has different etch requirements than another wafer having another pattern.
These differing etch requirements dictate that such processing parameters as process gas chemistry, temperature, pressure, flow rates and power also differ between etch processes. Moreover, these processing parameters for each etch process are usually precise. Accordingly, each etch process typically has a narrow process window. In fact, several specialized xe2x80x9crecipesxe2x80x9d of processing parameters exist for etch process, but determining these recipes can consume a great deal of time and cost. Therefore, there is a need to broaden these narrow process windows.
One reason these process windows are so narrow is because of competing etch and deposition mechanisms within the plasma. A process window is bounded by inadequate etch selectivity at one extreme and inadequate etch stop depth at the other extreme. At one extreme, the etch selectivity means that the etch process removes one type of material while leaving other materials unaffected. This selectivity may be to any material, such as selectivity to the photoresist or the underlying substrate. The etch selectivity is inadequate when surfaces that are not meant to be etched are etched. Moreover, inadequate etch selectivity may cause overetching of a hole depth or pattern and reduce yield.
Etch selectivity is often enhanced by introducing polymer precursors in the process gas. In general, a polymer precursor is contained within the process gas and the polymer is deposited on any surface where there is no oxygen coming off the surface. Conversely, when oxygen is coming off the surface, the oxygen tends to prevent a net deposition of polymer. Thus, the polymer deposits on the surfaces of non-oxygen containing materials and not on the surfaces of oxygen containing materials.
At the other extreme the process window is bounded by excessive deposition, or inadequate etch stop depth. If too much polymer precursor is permitted to deposit on the surface of non-oxygen materials, polymer deposition can occur on top of an oxygen-containing material and then etch stop occurs. Etch stop is the cessation of etching prior to the full etching, and usually occurs during the etching of holes in the workpiece due to an excess deposit of polymer in the hole. The depth at which the etch stop occurs is called the etch stop depth. Inadequate etch stop depth means that the etch stop depth is always less than the desired etch depth.
Between these two extremes the etch process has adequate etch selectivity and adequate etch stop depth. In other words, the workpiece has an adequate layer of polymer on the non-oxygen containing surface so that the surface is sufficiently protected from etching and there is a sufficient etch rate to etch the workpiece to a desired depth. However, because a process window is so narrow it is easy to diverge from the window. Too little polymer and inadequate etch selectivity occurs. Too much polymer deposition and inadequate etch stop depth occurs.
One example of an etch process with a narrow process window is a self-aligned contact (SAC) etch process. Typically, a photoresist mask is placed over an oxide layer to be etched. The oxide may be BPSG, undoped silicate glass (USG) or some other oxide and may have various layers of varying oxide materials. Furthermore, there is a silicon substrate with a bottom nitride layer overlying the substrate in order to isolate a poly line conductor from the substrate. The poly line is placed over the insulated substrate and capped with an insulating nitride layer. In addition, there may be various other layers over the poly line. The thickness of the nitride layer overlying the poly line can be as thin as 400 angstroms, while the overall thickness of the poly line insulating layer can be from less than 1 micron to 2 microns.
The SAC etch process requires that a hole be etched into the oxide layer down to the substrate. However, as the hole nears the substrate the poly line may occupy a part of the hole. The poly line insulating layer, including the nitride layer and possibly other materials and layers, are an etch stop layer. The purpose of this etch stop layer is to keep the etchant from xe2x80x9cblowing throughxe2x80x9d and exposing the poly line conductor. Later, a conductive material will be deposited within the hole and contact must be made with the substrate but not the poly line.
The nitride layer over the poly line must be protected with polymer to prevent etching, but the oxide must not be protected because it needs to be etched. in order to accomplish this, the processing parameters call for a relatively low source power and process gas flow. For example, for a process gas of C4F8, the flow rate is between 12 and 14 standard cubic centimeters per minute (SCCM). The size of the process window is a mere 1 SCCM. If the flow is increased by 2 SCCM etch stop will occur, and if the flow is decreased by 2 SCCM excessive etching will occur. Thus, the process window is so narrow that a change in the gas flow rate of less than about 10% and only 1 SCCM will put the gas flow rate out of the process window.
Another example of an etch process with a narrow process window is a high-aspect ratio etch process. In a high-aspect ratio etch process the aspect ratio, or ratio of the hole depth to the hole diameter, is large. in general, the greatest aspect ratio achievable with current technology is between 5:1 and 6:1 because etch stop tends to occur at higher aspect ratios.
For example, if the process gas contains polymer precursors (e.g., fluorocarbons and fluorohydrocarbons) and the chemistry is xe2x80x9cleaned upxe2x80x9d so that there is a lower carbon (C) to fluorine (F) ratio, this tends to form less polymer. This means that a deeper hole and higher aspect ratio hole should be able to be etched into the workpiece. The problem, however, is that at higher aspect ratios the photo-resist mask will be erode and lead to xe2x80x9cblowing outxe2x80x9d of the hole.
If, to avoid this problem, the process gas chemistry is made more xe2x80x9crichxe2x80x9d by increasing the flow and putting in more carbon rich chemistry and higher pressures, then a thin polymer layer is formed on the photo-resist. This permits better passivation of the photo-resist. However, more polymer is also deposited in the hole and at some point this excessive deposition of polymer will cause the hole to taper off and etch stop. Typically, if the source power is 10-20% too low etch stop will occur, and if the power is 10-20% too high the etch selectivity will be severely degraded. This narrow process window also applies to other process parameters such as gas flow rate and pressure.
In order to broaden these process windows, an inductively coupled plasma (ICP) reactor or a capacitively coupled plasma (CCP) reactor is often used. In general, the common elements of the two types of plasma reactors include a reactor chamber and a workpiece support within the chamber. The workpiece is placed on the support and a process gas is introduced into the chamber. A plasma is created by irradiating the process gas with an energy emission, such as from an electromagnetic energy source using an RF applicator (for example, an inductive RF antenna), to ignite and sustain the plasma within the reactor chamber. This energy emission breaks downs or dissociates the process gas into several chemically reactive species. Moreover, some of these species are ionized, giving them a net electric charge, which renders them maneuverable in the chamber by an electric field from a bias signal applied to the workpiece. This maneuverability allows, for example, a nearly perfect vertical etch on the workpiece by having the fragments impact the surface of the workpiece at a perpendicular angle.
Typically, an ICP reactor is employed when a high etch rate plasma is required. This is because the ICP reactor generally has a higher ion density and thus a higher etch rate. In general, however, etch selectively is degraded at higher etch rates. Thus, although an ICP reactor tends to have higher plasma density and higher etch rates, the etch selectivity tends to be better in a capacitively coupled reactor. Further, although a CCP reactor usually has better etch selectivity the plasma density and etch rates tend to be much lower than an ICP reactor.
One reason for the difference in etch selectivity between inductively coupled and capacitively coupled reactors is the amount of dissociation within the plasma. In particular, the plasma of an ICP reactor typically has more dissociation that a CCP reactor. Dissociation means that the molecules of the process gas are separated into two or more of the constituent parts of the molecules, generally by inelastic collisions with electrons. These constituent parts, or species, formed as a result of the dissociation, may contain atoms, ions or radicals. Depending on several processing parameters including the chemistry of the process gas, the source power and the chamber pressure, the species formed may be a bigger species or a smaller species. in general, a bigger species means that the species is a less-dissociated species and a smaller species means that the species is a more highly-dissociated species.
As mentioned above the plasma of inductively coupled and capacitively coupled plasma reactors dissociate differently. Specifically, in an ICP reactor the plasma tends to dissociate into relatively small species. Further, these small species can have an adverse effect on selectivity. For example, free fluorine (F) is a small species that is usually undesirable because it tends to etch most any material and thus degrades etch selectivity. On the other hand, the plasma in a CCP reactor tends to dissociate into bigger species. Generally, therefore, in a CCP reactor there is a larger density of bigger species and in an ICP reactor there is a larger density of smaller species. This is one reason why etch selectivity is usually better in a CCP reactor.
For example, in an ICP reactor with a process gas of CHF3, the irradiated process gas dissociates into several species including C, CH, F, C2, CF, CHF, CF2 and CHF2. The bigger, or less-dissociated species include CHF, CF2 and CHF2, while the smaller, or highly-dissociated species include C, F, CH, CF and C2. Thus, the bigger species more resemble the molecular structure of the process gas and the smaller species more resemble the atomic constituents of the gas.
FIG. 1A illustrates the breakdown for the CHF3 process gas in an inductively coupled reactor. The largest percentage (46%) was a relatively small species CF, the second largest percentage (38%) was a relatively big species CHF2, the remainder being 7% C, 6% CF2 and 3% miscellaneous species. The ratio, CHF2/CF=0.83, represents the ratio of bigger species to smaller species. Thus, the smaller species, CF, is the dominant species.
In fact, when the process gas is a fluorocarbon in an inductively coupled plasma reactor, such as CF4, C2F6 or C4F8, all dissociate so that CF is usually the dominant species. Although the amounts of free F and other species do change slightly, the ratios of the bigger species to smaller species changes very little. Therefore, what is needed is a method and an apparatus for actively controlling the density of the bigger species and smaller species generated within the plasma so that the ratio of the two can be increased or decreased. Of course, the atomic composition of the process gas will limit the species that can be generated, but controlling the densities of each species would affect the ratio. Increasing the ratio would tend to give better etch selectivity and decreasing the ratio would give better etch stop depth.
Several approaches for controlling the density of species generated within the plasma have been tried in an attempt to broaden the process window, but with only limited success. For example, although changing the chemistry of the process gas has yielded minor success, there have been no major breakthroughs. Further, adjusting other processing parameters such as the source power, bias power and bias frequency have shifted operating points but have not drastically broadened the process window.
One promising technique is to pulse or time-modulate the plasma. in this specification time-modulation and pulsing will be used interchangeably. FIG. 1B illustrates the dissociation of a CHF3 process gas under the same conditions as FIG. 1A, except that FIG. 1A is a continuous wave (CW), or unpulsed, plasma and FIG. 1B is a time-modulated, or pulsed, plasma. The source power was pulsed at 10 microseconds on and 10 microseconds off. The major species was CHF2 (62%), the next major species was CF (25%), with the remainder C (8%), CF2 (4%) and miscellaneous other species (1%). Moreover, the ratio of bigger species to smaller species, or CHF2/CF=2.48, has been increased dramatically.
Several approaches have been proposed regarding plasma pulsing. A majority of these approaches use capacitive coupled reactors. As stated earlier, one problem with capacitively coupled reactors is that they do not have the etch rate of an inductively coupled reactor. Consequently, a CCP reactor has the disadvantage that it cannot etch as deep or fast as an ICP reactor.
Other pulsing approaches use an inductively coupled reactor or a capacitively coupled reactor with a high antenna-to-wafer gap. One problem, however, with a high antenna-to-wafer gap is that the plasma density is degraded. This, in turn, degrades the etch rate. In addition, most approaches use reactors having chamber walls made of metal. Metal walls, however, tend to contaminate the plasma when the plasma comes in contact with the metal.
Accordingly, what is needed is a method and an apparatus for actively controlling the species generated within a plasma. Moreover, it is desirable that the method and apparatus have a high plasma density and etch rate associated with an ICP reactor and a high etch selectivity associated with a CCP reactor. Such a method and an apparatus would permit relatively deep holes to be etched in the workpiece and reduce workpiece processing time.
What is further needed is a method and an apparatus that will broaden the narrow process windows of etch processes such as SAC and high aspect ratio etch processes. Specifically, the method and the apparatus would permit active control of the density of species generated within a plasma so that the etch selectivity would be decoupled from the etch stop depth. This would allow both the etch selectivity and etch stop depth to be increased rather than one increasing while the other decreases. A broader process window would save cost and time on plasma etch processing and make the process easier to use.
What is further needed is a method and an apparatus that would allow control over the density of the species generated within the plasma such that the density of the species and the dissociation could be varied from that of a capacitively coupled reactor to that of an inductively coupled reactor. This would permit the user to selected a specific etch selectivity. For example, in the SAC etch process discussed above the etch selectivity would be high at the initial stages of the hole etch but be gradually increased as the depth of the hole increased. Thus, etch stop could be avoided as well as any delays and shut-downs associated with the need to change the processing parameters.
In addition, what is further needed is a method and an apparatus for controlling the density of species generated within a plasma wherein the reactor has a small antenna-to-workpiece gap. This reduced gap would enhance the plasma density at the surface of the workpiece. Moreover, what is needed is a method and an apparatus whereby the reactor chamber has non-metallic walls. This would eliminate contamination of the plasma due to metal interaction with the plasma.
Whatever the merits of existing methods and plasma reactors for actively controlling the species generated in a plasma, they do not achieve the benefits of the present invention.
To overcome the limitations in the prior art as described above and other limitations that will become apparent upon reading and understanding the present specification, the present invention includes method and an apparatus for actively controlling the density of the species generated in a plasma reactor using time-modulation.
The method and apparatus of the present invention permit control over the dissociation of the process gas by actively controlling the density of species generated within a plasma. In particular, the present invention uses time-modulation, or pulsing, to achieve this control without sacrificing adequate etch stop, plasma density or etch rate. Moreover, control over the density of the species generated allows the user to select an etch selectivity ranging from that of a capacitively coupled reactor to that of an inductively coupled reactor. This invention decouples the competing mechanisms of etch selectivity and etch stop depth and broadens the process window of several types of etch processes. Thus, the present invention can reduce processing time and increase yield and etch accuracy over existing techniques.
In a preferred embodiment, the present invention includes an apparatus having a non-metallic reactor chamber. In particular, the reactor has a reactor chamber including an interior surface and walls made of a non-metallic material. Further, the reactor has a workpiece support for holding a workpiece within the reactor chamber, a ceiling above the workpiece support and a RF applicator that couples power from a power source into the reactor chamber. A signal modulator can be coupled to the power source, a bias source, or both and provides a means for modulation.
A method of the present invention includes a method of processing a workpiece in this reactor. The method further includes generating a plasma within the reactor chamber by irradiating a process gas with an energy emission from the RF applicator. In addition, the method includes modulating the energy emission to control the density of the species generated within the plasma.
Another preferred embodiment includes a reactor having solenoidal inductive antenna. The reactor includes a reactor chamber, a workpiece support having a support plane, a reactor enclosure and a non-planar inductive antenna next to the reactor enclosure. The non-planar inductive antenna includes inductive elements that are spatially distributed in a non-planar manner relative to the support plane. Moreover, the reactor includes a signal modulator for coupling to a power source, a bias source or both, as a modulation means.
A method of this invention includes providing a reactor having a solenoidal inductive antenna. Moreover, the method includes generating a plasma by irradiating a process gas with an energy emission from the non-planar inductive antenna. The method further includes modulating the energy emission so as to control the density of the species generated within the plasma. Preferably, control of the species is accomplished by varying the on-time and the off-time of the modulation.
Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not the preceding summary or the following detailed description.